Derivative Spectroscopy and its Application at Detecting the Weak Emission/Absorption Lines

The development of spectroscopic survey telescopes like Large Sky Area Multi-Object Fiber Spectroscopic Telescope(LAMOST),Apache Point Observatory Galactic Evolution Experiment and Sloan Digital Sky Survey has opened up unprecedented opportunities for stellar classification.Specific types of stars,such as early-type emission-line stars and those with stellar winds,can be distinguished by the profiles of their spectral lines.In this paper,we introduce a method based on derivative spectroscopy(DS)designed to detect signals within complex backgrounds and provide a preliminary estimation of curve profiles.This method exhibits a unique advantage in identifying weak signals and unusual spectral line profiles when compared to other popular line detection methods.We validated our approach using synthesis spectra,demonstrating that DS can detect emission signals three times fainter than Gaussian fitting.Furthermore,we applied our method to 579,680 co-added spectra from LAMOST Medium-Resolution Spectroscopic Survey,identifying 16,629 spectra with emission peaks around the Hαline from 10,963 stars.These spectra were classified into three distinct morphological groups,resulting in nine subclasses as follows.(1)Emission peak above the pseudo-continuum line(single peak,double peaks,emission peak situated within an absorption line,P Cygni profile,Inverse P Cygni profile);(2)Emission peak below the pseudo-continuum line(sharp emission peak,double absorption peaks,emission peak shifted to one side of the absorption line);(3)Emission peak between the pseudo-continuum line.

Type Ia Supernova Explosions in Binary Systems:A Review

Type Ia supernovae(SNe Ia)play a key role in the fields of astrophysics and cosmology.It is widely accepted that SNe Ia arise from thermonuclear explosions of white dwarfs(WDs)in binary systems.However,there is no consensus on the fundamental aspects of the nature of SN Ia progenitors and their actual explosion mechanism.This fundamentally flaws our understanding of these important astrophysical objects.In this review,we outline the diversity of SNe Ia and the proposed progenitor models and explosion mechanisms.We discuss the recent theoretical and observational progress in addressing the SN Ia progenitor and explosion mechanism in terms of the observables at various stages of the explosion,including rates and delay times,pre-explosion companion stars,ejecta–companion interaction,early excess emission,early radio/X-ray emission from circumstellar material interaction,surviving companion stars,late-time spectra and photometry,polarization signals and supernova remnant properties.Despite the efforts from both the theoretical and observational sides,questions of how the WDs reach an explosive state and what progenitor systems are more likely to produce SNe Ia remain open.No single published model is able to consistently explain all observational features and the full diversity of SNe Ia.This may indicate that either a new progenitor paradigm or an improvement in current models is needed if all SNe Ia arise from the same origin.An alternative scenario is that different progenitor channels and explosion mechanisms contribute to SNe Ia.In the next decade,the ongoing campaigns with the James Webb Space Telescope,Gaia and the Zwicky Transient Facility,and upcoming extensive projects with the Vera C.Rubin Observatory's Legacy Survey of Space and Time and the Square Kilometre Array will allow us to conduct not only studies of individual SNe Ia in unprecedented detail but also systematic investigations for different subclasses of SNe Ia.This will advance theory and observations of SNe Ia sufficiently far to gain a deeper understanding of their origin and explosion mechanism.

The Binarity of Early-type Stars from LAMOST medium-resolution Spectroscopic Survey

Massive binaries play significant roles in many fields.Identifying massive stars,particularly massive binaries,is of great importance.In this paper,by adopting the technique of measuring the equivalent widths of several spectral lines,we identified 9382 early-type stars from the LAMOST medium-resolution survey and divided the sample into four groups,T1(~O-B4),T2(~B5),T3(~B7)and T4(~B8-A).The relative radial velocities RVrelwere calculated using Maximum Likelihood Estimation.The stars with significant changes of RVreland at least larger than 15.57 km s-1 were identified as spectroscopic binaries.We found that the observed spectroscopic binary fractions for the four groups are 24.6%±0.5%,20.8%±0.6%,13.7%±0.3%and 7.4%±0.3%,respectively.Assuming that orbital period(P)and mass ratio(q)have intrinsic distributions as f(P)∝Pπ(1<P<1000 days)and f(q)∝qκ(0.1<q<1),respectively,we conducted a series of Monte-Carlo simulations to correct observational biases for estimating the intrinsic multiplicity properties.The results show that the intrinsic binary fractions for the four groups are 68%±8%,52%±3%,44%±6%and 44%±6%,respectively.The best estimated values forπare-1±0.1,-1.1±0.05,-1.1±0.1 and-0.6±0.05,respectively.Theκcannot be constrained for groups T1 and T2 and is-2.4±0.3 for group T3 and-1.6±0.3 for group T4.We confirmed the relationship of a decreasing trend in binary fractions toward late-type stars.No correlation between the spectral type and orbital period distribution has been found yet,possibly due to the limitation of observational cadence.

On the Rotation Properties of a Post-explosion Helium-star Companion in Type Iax Supernovae

Recent studies have suggested that type lax supernovae(SNe lax) are likely to result from a weak deflagration explosion of a Chandrasekhar-mass white dwarf in a binary system with a helium(He)-star companion.Assuming that most SNe Iax are produced from this scenario,in this work we extend our previous work on the threedimensional hydrodynamical simulation of ejecta-companion interaction by taking the orbital and spin velocities of the progenitor system into account.We then follow the post-impact evolution of a surviving He-star companion by using the one-dimensional stellar evolution code MESA.We aim to investigate the post-explosion rotation properties of a He-star companion in SNe Iax.We find that the He-star companion spins down after the impact due to the angular-momentum loss and expansion caused by the mass-stripping and shock heating during the interaction.This leads to the situation where the surface rotational speed of the surviving companion can drop to one-third of its pre-explosion value when it expands to a maximum radius a few years after the impact.Subsequently,the star shrinks and spins up again once the deposited energy is released.This spin-switching feature of the surviving He-star companions of SNe Iax may be useful for the identification of such objects in future observations.

FAST unveils a new era in the discovery of long-period pulsars in globular clusters

Globular clusters(GC)are among the oldest celestial bodies in the Milky Way.Typically possessing millions of stars and have been going through abundant stellar interactions,GCs are futile ground for hunting for compact objects,particularly,neutron stars.A significant number of fast-spinning millisecond pulsars(MSPs)have been discovered,comprising the majority of known GC pulsars.

Overview of the LAMOST survey in the first decade

The Large Sky Area Multi-Object Fiber Spectroscopic Telescope(LAMOST),also known as the Guoshoujing Telescope,is a major national scientific facility for astronomical research located in Xinglong,China.Beginning with a pilot survey in 2011,LAMOST has been surveying the night sky for more than 10 years.The LAMOST survey covers various objects in the Universe,from normal stars to peculiar ones.

LAMOST’s view on the Gaia-Sausage-Enceladus merger event

A combination of massive spectroscopic surveys and Gaia DR2 has brought light into the chemical and kinematical properties of a large number of stars,thus building up a mature picture of the Gaia-Sausage-Enceladus (GSE) galaxy and unraveling its impact on the reconstruction of the history of our Galaxy.Based on the largest spectroscopic data set established in the LAMOST survey [1,2],Profs.